Area Forecast Discussion
Issued by NWS Wilmington, NC

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126
FXUS62 KILM 211441
AFDILM

Area Forecast Discussion
National Weather Service Wilmington NC
1035 AM EDT Thu Sep 21 2017

.SYNOPSIS...
Weak high pressure across the Great Lakes will control our
weather through the weekend. Tropical Storm Jose off the Mid-
Atlantic coast should weaken, while Major Hurricane Maria will
bring increasing ocean swells and strengthening rip currents to
the beaches by Friday. Maria is expected to move north and
remain offshore of the Carolinas Monday and Tuesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1030 AM Thursday...No changes being made at this time. With
the 12Z WRF I was hoping to have a reason to raise POPs a bit with
such cool temps aloft today. However there is a pretty obvious
convective feedback issue in the model this evening. This is capping
afternoon convection and then focusing it in a spurious looking
manner this evening. Will stick with the higher res guidance like the
HRRR that shows convective initiation by about 19Z. The last run or
two show some sense of focus along the sea breeze. May adjust the
forecast accordingly especially if supported by the 12Z GFS.

As of 300 AM Thursday...Mid level low pressure and or troughing
will continue to linger across the area through the period. At
the surface it will be a weak trough or convergence boundary
aligned mostly inland. These features sparked a decent coverage
of showers and thunderstorms Wednesday ans see no reason why the
same wouldn`t today. I have increased the pops slightly for
today mainly along the northern tier of counties where the best
coupled forcing will reside. As for highs, I opted for the
mostly warmer MET numbers which performed slightly better
Wednesday. We should see a few more sites meet or eclipse the
ninety degree mark. Lows Friday morning will mostly be in the
upper 60s to lower 70s.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
As of 300 AM Thursday...A convoluted upper air pattern will
exist Friday and Saturday as a 500 mb high sits over the Great
Lakes, while a shear zone extending southwestward from Tropical
Storm Jose closes off into an upper low over southern Georgia
and Alabama. The surface pattern is simpler as there is no
reflection of the upper low, only the broad high pressure over
the Great Lakes which remains in place through the period. In
the western Atlantic, Tropical Storm Jose is expected to
continue weakening and should lose its remaining tropical
characteristics by Friday afternoon or evening. Hurricane Maria
should turn northward near 72W longitude into the upper trough
created by TS Jose. If it weren`t for Jose, the Great Lakes
upper ridge would likely bridge across to the Bermuda ridge well
offshore, steering Maria into the Southeastern U.S.

Friday afternoon could see scattered showers and t-storms
develop along the Grand Strand coast, pushing west-southwestward
toward Florence and Kingstree by late in the day. Seabreeze-
enhancement to low-level convergence looks to be the key for
this mesoscale precipitation event which appears to be
reasonably handled by the 3km NAM and RGEM models. By Saturday
drier air in the northeasterly flow should make showers much
more unlikely to develop.

Low-level east to northeasterly winds should drop temperatures
down a couple of degrees from the unseasonably warm temps of the
past few days. 850 mb temps around +14C (compared to +17C
Wednesday) means highs should only reach the lower 80s at the
coast both days with 86-87 expected inland.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
As of 330 PM Wednesday...If NHC Maria forecast tracks hold, the
extended forecast will feature a healthy dose of warm late
summer weather and mainly dry, with daytime temperatures running
2-5 degrees above normal for this time of the year. Dry air
aloft remains in place of an extent that only showers are
advertised in the extended period. The exception is Tuesday when
we have the deepest moisture in place as Maria passes between
the Carolinas and Bermuda, an outer band may fling ashore.
Otherwise mainly dry and warm this period.

&&

.AVIATION /12Z THURSDAY THROUGH MONDAY/...
As of 12Z...A weak trough lying across the eastern Carolinas
will feature broadly converging surface winds across the area
today. This by itself would probably produce some towering
cumulus this afternoon and maybe an isolated shower or two.
However unusually cold temperatures aloft associated with an
upper level trough will help produce scattered showers and
thunderstorms this afternoon. Activity should begin developing
around 18Z and should last through sunset. Until specific storm
cells begin developing I have addressed the thunderstorm
potential with VCTS/VCSH in the aviation forecasts. Light
surface winds will remain generally N/NW inland, but will turn
E/SE near the coast this afternoon with the seabreeze.

VFR conditions are expected tonight after the day`s convection
dies away, however with light winds and mainly clear skies
ground fog is expected to develop late. Visibilities are
currently progged to fall to 3-5 miles, however this could be
dropped lower in later forecasts depending on precisely where
showers and t-storms develop later today.

Extended Outlook...Mainly VFR conditions through the period are
expected with the exception of a few hours of early morning low
clouds or fog most mornings through the period. The highest
risk of IFR conditions will be in the 0900-1200Z timeframe each
morning.

&&

.MARINE...
NEAR TERM /THROUGH TONIGHT/...
As of 300 AM Thursday...A light south to southwest wind is currently in
place across the waters. There should be a brief westerly turn to
the winds later this morning before the southerly component returns
later mostly driven by the sea breeze and the enhancement of the
inland trough. Speeds throughout should be ten knots or less. Seas
are mostly 2-3 feet and should remain so with a more significant
swell component arriving just after the near term period.

SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
As of 300 AM Thursday...Weak high pressure will remain situated
over the northern Mid-Atlantic states Friday through Saturday,
while offshore a pair of tropical systems (Jose and Maria)
swirl across the western Atlantic Ocean. For Friday and Saturday
our winds will be driven primarily by the weak high, with NE-E
winds around 10 knots. By Saturday evening the broad outer
circulation of Hurricane Maria will begin to increase NE winds
to around 15 knots.

Maria`s long-period swells will impact the area well ahead of
any increase in wind. Dominant wave periods around 15 seconds
will slowly increase in size and could reach 6 feet as early as
daybreak Saturday. Look for the issuance of Small Craft
Advisories this weekend.

LONG TERM /SUNDAY THROUGH MONDAY/...
As of 330 PM Monday...Increasingly dangerous marine conditions
this period as Maria generates large SE swell, as she moves
north, east of the Bahamas. If NHC forecast tracks hold, we can
expect increasing NE and N winds late Sunday and Monday but
probably below Gale force through Monday. Even if winds do not
become strong this period, we will still likely require a
headline for large swell-heights. This will produce inlet
turbulence during the outgoing tides as well, and larger than
breakers near ocean sandbars, while extending the width of the
surf zone. Expect seas of 6-10 feet, mainly in SE swell by late
Monday.

&&

.ILM WATCHES/WARNINGS/ADVISORIES...
SC...None.
NC...Coastal Flood Advisory from 10 AM this morning to noon EDT
     today for NCZ107.
MARINE...None.

&&

$$
SYNOPSIS...TRA
NEAR TERM...MBB/SHK
SHORT TERM...TRA
LONG TERM...MJC
AVIATION...TRA
MARINE...



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